This invention concerns STACKABLE ELECTRONIC EQUIPMENT ASSEMBLY WITH IMPROVED CIRCUIT BOARD COOLING ARRANGEMENT.
For many years it has been common practice to design and build professional electronic equipment to fit cabinets that conform to internationally agreed dimensions. These cabinets, with a standard width of nineteen inches, can be stacked vertically, in multiples of one unit of 1.75 inches, in nineteen inches wide racks and cubicles specifically made for this purpose. These `19 inch` racks and cubicles have side flanges pre-drilled with a pattern of holes repeated every 1.75 inches (1`U`) to simplify the fixing of any vertical combination of `rack-mount` equipment. Each piece of `rack-mount` equipment has corresponding side flanges pierced with bolt holes at N`U` intervals that align precisely with the pattern of holes in the cubicle flanges. This internationally agreed format permits the assembly of any configuration of cabinets of various sizes to be stacked one above the other, limited only by the available height in a standard `19 inch` wide cubicle.
To accommodate the increasing complexity of electronic equipment it has become common practice to assemble large quantities of electronic components and integrated circuits onto a plurality of printed circuit boards (PCBs) that plug into a suitable cabinet, vertically like books on a bookshelf, or horizontally in layers one above the other, in such a way that each PCB may be removed quickly for servicing or replacement, preferably from the front of the equipment cabinet whilst it remains rack-mounted in the cubicle.
An assembly of vertical PCBs has the advantage that heat dissipated by the circuit components on each PCB rises by natural convection between the PCBs, assisting the cooling process and helping to avoid localised heating that can lead to premature component failure. In all but the smallest examples, cooling must be enhanced by fitting electric fans to the equipment, forcing air to pass between the PCBs, to assist in the removal of heat from the cabinet. With vertically mounted PCBs, forced air cooling usually requires a plenum above and sometimes also below the array of PCBs to form ducts for the cooling air supply and hot air exhaust. This unfortunately adds to the height of the equipment cabinet consuming more of the vertical space available in a rack-mount cubicle. A further disadvantage of an assembly of vertically mounted PCBs is that the maximum number of PCBs that can be accommodated in such an equipment is ultimately determined by the available internal width of a `19 inch` cabinet. A less complex equipment using fewer PCBs with this vertical PCB format suffers the disadvantage of inefficient use of the same volume of `rack-mount` space, since the cabinet must still occupy the full `19 inch` standard width.
A solution to these problems can be found in using an arrangement of PCBs in the horizontal plane, stacked vertically one above the other. If a PCB size is chosen to suit the dimensions of the standard `19 inch` cabinet, then differing numbers of such PCBs can be stacked according to the complexity of the equipment in cabinets that increment in height by multiples of `1U`. Natural convection cooling has little beneficial effect on the circuit components on each horizontal PCB and serious local heating problems will be encountered unless effective forced air cooling is provided. Filtered air may be supplied conveniently through the front of the cabinet where access is needed as well for removal of PCBs for normal maintenance, but exhausting the heated air from the rear of the PCBs is normally precluded by the need for an interconnection `mother board` across the rear of all of the plug-in horizontal plane PCBs. Lateral forced air cooling from side to side of the equipment cabinet is sometimes attempted, but this method fails when cabinets are rack-mounted in adjacent `19 inch` cubicles, or are field operated in so-called `flight cases` with only front and rear access covers removed.